1. Field of the Invention
The present invention relates to an optoelectronic high-precision displacement sensor and more especially a sensor of this type using differential action photoreceivers.
2. Description of the Prior Art
Optoelectronic sensors employing differential photoreceivers for measuring machining tolerances and for measuring displacements are known.
In U.S. Patent application Ser. No. 937,688 of Aug. 28th, 1978 in the name of the present applicant there is described a device for detecting whether a measured dimension deviates or does not deviate from a nominal value by a predetermined increment. This device comprises a feeler probe for following the surface of a mechanical component, a variable surface window having an area controlled by the feeler probe, two pairs of photoelectric receivers and an electronic circuit including three flip-flops. The receivers of each pair detect differential illumination projected by a light source through the window. The first flip-flop is activated when the differential illumination from a certain pair of photoreceivers exceeds a positive predetermined value; the second flip-flop is activated when the differential illumination from a second pair of photoreceivers exceeds a negative predetermined value and the third flip-flop is activated when neither the first nor the second flip-flops is activated, which implies that the dimension tested lies within the nominal limits.
Such a device is designed for measuring tolerances, but not dimensions, with the high level of accuracy required.
Optoelectronic dimension or displacement measuring devices are described in French Pat. No. 2,300,367 of Feb. 7th, 1975 and in French Rider Pat. No. 2,340,574 of July 26th, 1976 both in the name of the current applicant. In these patents, a pair of photoreceivers have a sensitive surface with a square or rectangular contour (formed, in the specific embodiments by a fixed cover in the event photoreceivers include a round focusing lens). The photoreceivers are connected in series by having a common electrode between two electrical supply lines. The measurement of a voltage variation at the common electrode due to a displacement of a movable cover linked to the stem of a probe, which partially masks each photoreceiver, is indicated by an electrical or digital display indicator.
Such a system can be used for dimensional measurements. In this case, light sources are associated with each of the two photoreceivers. The photoreceivers are separated from each other by being located on one and the same side with respect to the moving probe system and on either side of a mask. The mask is a bar linked to the moving stem. The mask partially covers each of the photoreceivers to intercept the light emitted by the sources in such a manner that when the illumination of one photoreceiver decreases, that of the other increases.
This prior art arrangement is relatively bulky because the two photoreceivers are: (1) spaced relative to each other and, (2) relatively distant from the moving stem. These factors lead to the use of two separated light sources, one per photoreceiver. The same brightness must, therefore, be derived from the two sources, which is difficult.
Furthermore, this prior art arrangement does not make it possible to reach a high degree of accuracy, of approximately 0.1 .mu.m for a dimension of 1 mm which the device of the present invention permits. Inevitable sideward mechanical play in the probe stem in guide sleeves thereof leads to unequal variations in the amount of light received by the photoreceivers. The differences of these variations do not exactly cancel when the signals resulting from the differential illumination from photoreceivers are subtracted.